The aim of the proposed research is to determine the X-ray structures of eukaryotic translation initiation factors eIF2 and IF2 and of the complex of eIF2 with Met-tRNAi. Eukaryotic initiation factor 2 (eIF2) and initiation factor 2 (IF2) are structurally dissimilar factors, which are both involved in promoting the binding of the initiator tRNA to the ribosome. It has long been known that in one of the early steps of the translation initiation pathway, eukaryotic initiation factor 2 (eIF2) is responsible for recognizing, binding and positioning Met-tRNAi at the P-site on the ribosome and also for influencing start codon selection). Under a variety of conditions of cellular stress, like nutrient deprivation or viral infection, the cont... binding of the initiator tRNA to the ribosome with the help of eIF2 can become the rate limiting step in translation initiation. Initiation factor 2 (IF2) is a surprising recent addition to the plethora of general translation initiation factors in eukaryotes. It was discovered in yeast and named IF2 because of its homology with the prokaryotic initiation factor IF2, which is known to promote the binding of fMet-tRNA to the ribosome in prokaryotes. Using a combination of molecular genetics and biochemical studies, the Saccharomyces cerevisiae IF2 homolog was shown to function in general translation initiation by promoting Met-tRNAi binding to ribosomes. Cont.... By combining the exquisitely detailed structural data from the X-ray structure with the biochemical and genetic studies performed by other groups, the central question of how eIF2 recognizes, binds the tRNAi and places it on the ribosome to start the synthesis of the protein chain at the correct start codon will be addressed. Because of its recent discovery, not much biochemical data is available on initiation factor IF2. A three dimensionall structure of IF2 should shed light on the role of this factor in the complex process of translation initiation.